feat: GET /rag/documents and DELETE /rag/documents/{id}

[PurviMalhotra]

Summary

Closes #63
Added RAG document management endpoints. (new pr due to issues with the last fork)

Changes include:

• Added GET /api/v1/rag/documents to list ingested RAG documents with metadata.
• Added DELETE /api/v1/rag/documents/{id} to remove a document and rebuild the FAISS index from remaining stored documents.
• Added RAGDocument DB model plus Alembic migration for storing document metadata.
• Updated RAG ingest flow to persist uploaded PDFs and metadata alongside the FAISS index.
• Added RAG_DOCUMENT_STORAGE_PATH config.
• Added tests for RAG document listing, deletion, and index rebuild behaviour.
• Updated API docs for the new endpoints.
• Updated alembic migration file's down_revision to current head.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor
• Tests
• Infra / CI

Checklist

• I have read CONTRIBUTING.md
• My code follows the project style (PEP 8 for Python, ESLint for TS)
• I have added/updated tests where relevant
• pytest backend/tests/ passes locally
• I have not committed .env or any secrets
• I have updated documentation if needed